I just read Ed Creeger's comments on propping the trainer with the SC.40 (whatever the SC.40 is). Ed's comment indicated that a 10x6 prop would fly the plane, but wouldn't offer the braking that the 11x5 would. Nor would the
10x6 get the plane off the ground as quickly. This is my chance to learn something about propping efficiently (effectively?).
I would have thought that 10x6 would get the plane moving down the runway quickly and at a faster pace. What is it about the 11 that would make the 5 faster than the 6? Is it the length which gives the prop more "lift"? If the
6 (theoretically) moves the plane 6 inches in one revolution and the 5 moves it 5 inches in one revolution, I would think the 6 would move it faster. Obviously, I'm not thinking of this correctly. I'd appreciate some discussion/comments on the issue.
Ed, why do 2 strokes live longer lives when permitted to rev up a bit?
An SC engine is just a Sanye engine that is made in China. I like them.
In the US, they used to be known as the ASP brand of engines. Indy R/C used to import them into the US.
Global Hobbies (Hobby People) are now importing them into the USA as Magnum engines.
England (and maybe parts of Europe - I don't know) import the same engines with SC cast into the bypass port side of the engine as their brand name (Super Custom).
They are, for all practical purposes, the same engines. All good.
Pitch in a propeller is analagous to the gears in our car transmissions. The lower the pitch number, the lower the gear we are using in the car. We all know that if we start in first gear in a car, we will accelerate faster, but have a lower top speed than if we used a higher gear. This analog works very well for explaining how pitch affects the pull and thrust of a model airplane propeller. In a sense, the diameter of the prop can be viewed as the size of the rear tires (or front, if you please) that are receiving the power output of the engine. Larger tires put my power on the ground. Let's assume for this argument that the word "large" is being applied to the width of the tires. Not the diameter.
Higher pitch and higher diameter increase the load upon the engines we use, whether two-stroke, four-stroke or electric. Just as the timing of an electric motor can be changed in order to optimize the most power output to power consumed ratio, our IC engines are "tuned" to operate best within certain rpm ranges. Most sport two-stroke engines are tuned to operate over
10,000 rpm. Some much higher. Our engines should be allowed to rev into their power band in order to extract the most power and to help them create less heat. Heat is the killer for all IC reciprocating style engines.
Today we are beginning to see a migration of two-stroke engine tuning toward lower rpm operation. Why? It is quieter and it optimizes the engine's tuning for turning larger, higher load propellers, such as those being used in 3D flying. Fitting a Webra .50 GT with a 12" prop for 3D flying has proven not to be the best use for this engine. It was designed to scream. Let it scream.
I bought a Rossi .45 3D engine a while back. What's different? It's tuning/porting/timing is optimized for swinging the aforementioned larger 3D props. Everyone associates the Rossi name with high rpm engines. I predict that this new Rossi is just the beginning of a new trend in lowering the operating power band to optimize the two-stroke's performance for 3D flying.
Cooler operation is better. That's why I say to let your high rpm designed two-stroke rev up. It is quite possible that these engines will become extinct in the not to distant future.
All of this is just my personal opinion and is not presented as gospel. I'll be happy to listen to any arguments, as long as they remain civil.
This is an addendum to my just sent previous response to your questions.
Our glow engines have somewhat fixed timing. Running a larger prop with timing that is optimized for a smaller prop (glow plug, fuel, mixture, compression) will advance the timing. This makes the engine run hotter if the larger prop presents more of a load to the engine. Hot is bad.
One can run the engine a bit richer to retard the timing, if the jump in size is not too great. Or, one can change glow plugs to something more suitable for the given load. Or one can add a head shim to decrease the compression. Decreasing the compression effectively retards the timing and lowers the operating temperature. Good. Lowering the nitro content of the fuel will also retard the timing, but you will lose a little power. The head shim or lower heat range glow plug, while maintaing the nitro content is the best choice in most instances.
The angle of attack on the propeller's blades are the highest at the start of the takeoff roll. A high angle of attack equals a higher drag figure, so that a prop with a higher pitch won't allow the engine to run up closer to its rated RPM and horsepower output will be lower until the forward speed is much higher and the prop's AOA drops. The prop's blade length determines, to a large extent, its efficiency at lower airspeeds. The longer prop, with a lower pitch, will generate more thrust at a given RPM than a shorter one having a higher pitch, since tip vortices affect a smaller percentage of the span and the resulting drag is less. It's much more efficient to move a large column of air at a lower velocity than a small one at a high velocity. The designers of seaplanes, helicopters, turboprop airliners and high-bypass turbofan engines are well aware of this.
Thanks to Ed and Dan, I think I'm beginning to get a handle on the propping. I also found an article in "Getting Started in Radio Control Airplanes" that might help me understand this issue better. Still have to read it, but comments like yours will help me to grasp it easier.
While all of the above have been excellent posts, there is one gage that I have been told regarding props. A measure of their "power" is the pitch times the diameter. a 10x6 prop = 60. To match another prop of similar "power" you could use an 11x5. 11x5=55, close to 60. a
12x5 would also give you 60, but at some point, the diameter won't work on a given plane due to the landing gear hight.
On a full scale plane, you set a low pitch for takeoff, and a higher pitch for cruise. On our planes, I have tried 10x6 props on an OS40FX, and then the 11x5. The 11x5 is much better for takeoff and landing, at a small sacrifice in top speed. If you routinely orbit the pattern at
1/2 - 3/4 throttle, that top speed thing is moot. Hope this helps a little.
Another point to remember is that if you are flying a draggy, slow plane such as a biplane with a lot of struts, big landing gear and large frontal area, the plane won't allow a high pitch prop to go forward as fast as it wants to at full throttle.
Therefore, a pylon racer with a 2 stroke 40 might perform well with a
9x8, and a biplane with the same 2 stroke 40 might perform better with an 11x5.
Actually, from an electric perspective,where gears allow anything, coarse pitched props of large diameter going slow are often the best.
Use the RPM to match to the plane speed. Gives very efficient cruise and good top speed. And the large diameter gives good thrust on lower powers as well.
Sadly not relevant to ungeared IC engines and other dinosaur technologies ;-)
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